The respiratory rate has proven to be a good indicator of the deterioration of the condition of a patient. In combination with other vital signs, the respiratory rate plays a crucial role in early warning systems. Therefore, there is a need for continuous and reliable monitoring of a respiration signal, from which the respiratory rate is extracted, in intensive care units of hospitals. A similar need is present in the general ward setting of hospitals and in home healthcare applications, such as in telemedicine and chronic disease management.
While continuous monitoring of the respiration signal is available on bed side monitors for intensive care patients, various portable sensor systems are being developed to allow unobtrusive and prolonged measurement and monitoring of the respiration signal of mobile patients in general wards and in home healthcare settings with minimal discomfort.
Respiration monitoring can be based on different principles: the measurement of respiratory effort, for example thorax impedance plethysmography, respiratory inductance plethysmography, accelerometers, photoplethysmography or the measurement of respiratory effect, for example sound recording, temperature sensing, carbon dioxide sensing. Some sensors are already well established to monitor respiration. In intensive care units for example, thorax impedance plethysmography is the method of choice, whereas in sleep studies respiratory inductance plethysmography, often referred to as respiration band or respiband, is also commonly used. For ambulatory patients, such as on the general ward or in home healthcare, these sensors have limitations. A respiration band, for example, is considered to be too obtrusive and cumbersome by both medical personnel and patients.
A respiration monitoring system based on a multi-axial accelerometer overcomes these disadvantages. A multi-axial accelerometer is a device that measures an acceleration in multiple sensing axes. By evaluating the acceleration due to gravity with the different sensing axes, an accelerometer can be used as an inclinometer. The accelerometer is applied to an abdomen or a chest of a subject. The measured time-varying inclination reflects the abdomen or chest movement caused by respiration. This technique requires reliable signal processing to enable reliable monitoring under different conditions and postures of the patient.
Motion artifact is a well-known problem in patient monitoring. A motion artifact refers to a contamination of a physiological signal and a degradation of the measurement quality caused by physical activities of a patient, such as posture change, movement and talking Motion artifacts are more pronounced in a general ward setting than in an intensive care unit setting, since patients in the general ward setting generally have a more mobile activity pattern and are monitored most of the time without constant nursing surveillance, thus lacking knowledge on the presence of the physical activities and the measurement context. The problem becomes even more severe when monitoring patients in home healthcare settings.
Thus, if a multi-axial accelerometer is used to measure the respiratory rate in ambulatory conditions, such as home healthcare or a general ward, the accelerometer signals do not only change due to the respiration of a person but the accelerometer signals are also affected by unwanted motions that are not caused by respiratory motions.
Motion artifacts with frequency components that are different from breathing frequencies can be suppressed by straight-forward filtering in the frequency-domain. However, some of these unwanted motions, which may have a frequency component in the same range as the respiration, i.e., 0.1 Hz to 2 Hz or 6 respirations per minute to 120 respirations per minute, cannot be suppressed with a filter with a fixed frequency response.
US 2012/0296221 A1 discloses a method and apparatus for determining a respiration signal using an accelerometer. A vector magnitude of the accelerometer signals is evaluated to identify any unwanted or non-respiratory motion contributions to the acceleration signals. For a static, i.e. non-moving, multi-axial accelerometer the vector magnitude is always the same irrespective of the orientation of the sensor. If the position of the center-point of the tri-axial accelerometer changes due to whole body movements, for example walking, however, this causes an additional inertial acceleration component in addition to the acceleration due to gravity. A non-respiratory, inertial motion contribution is identified and then used to suppress and adaptively filter this unwanted motion contribution from at least one of the accelerometer signals. From the at least one filtered accelerometer signal, a respiration signal is determined that reliably and accurately represents the respiration of the subject.
US 2011/0066041 A1 discloses a respiration monitoring device including an accelerometer for application to the chest, whereby acceleration is possible due to both non-respiratory body motion and respiration, and an electronic circuit responsive to an acceleration signal from the accelerometer and operable to separate from the acceleration signal a heart signal, a respiration signal, and a substantially non-respiration body motion signal.
US 2011/0021928 A1 discloses noninvasive methods and systems of determining and monitoring an individual's respiration pattern, respiration rate, other cardio-respiratory parameters or variations thereof. In an embodiment, a single, miniature and chest-worn accelerometer is utilized to capture respiration-dependent parameters.
WO 2013/106700 A1 discloses systems and methods for determining mental and physical health using multi-scale metrics.